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  1. Member
    Join Date: Aug 2011
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    I want to correct the Display Aspect Ratio (DAR) of some of my video's from 16x9 to 4x3 without re-encoding.

    With the help of Mp4tools I can change the Pixel Aspect Ratio (PAR), but I don't know how to calculate the correct ratio.
    Can anyone help me with this?
    8 or 9 for heigth and 11 for width seems to be a good combination. But it's just an estimation.

    The mp4's are in PAL 1024 x 576. H.264.

    http://files.videohelp.com/u/203833/sample.mp4
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  2. Member
    Join Date: Mar 2008
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    That's a 4x3 video unnaturally stretched to be wider. Why not just use mkvmerge and set the aspect to 4:3 ?
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  3. Member
    Join Date: Jun 2012
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    Fortunately, someone worked this out about 20 years ago.

    4:3=1.0940
    16:9=1.4587
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  4. Member Cornucopia's Avatar
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    And the formula is:
    Code:
    DAR = (H rez / V rez) * PAR
    Scott
    "When will the rhetorical questions end?!" - George Carlin
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  5. Member
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    Davenext,
    Thank you for mentioning mkvmerge. But mkvmerge only wraps the mp4 into a 16x9 MKV container.
    I prefer to change the aspect ratio at the stream level. Mp4tools can do this.

    smrpix,
    1.0940 is not what I can fill in (see below).

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    Last edited by HitTheRoad; 10th Jun 2014 at 19:58.
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  6. Member
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    Cornucopia,
    I'm afraid I don't get the picture. Please explain.
    Last edited by HitTheRoad; 10th Jun 2014 at 19:36.
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  7. Member
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    Any idea what I'm supposed to fill in?
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  8. Member
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    Well, since it's comparing width to height rather than storage width to display width it's either

    4:3

    or

    0.75:1 (75:100/3:4)

    It depends on what the program thinks 'Pixel Aspect Ratio' means.
    Last edited by ndjamena; 10th Jun 2014 at 21:13.
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  9. I'd guess that in theory, the aspect ratio should be 4:3 ITU, which means it's around 1.367:1, rather than 1.333:1, which would make the pixel aspect ratio 10:13. If you want it to be exactly 1.333:1, then the pixel aspect ratio would be 3:4.
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  10. Member Cornucopia's Avatar
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    Don't get the picture? Wow, I would have thought that was pretty elementary.

    OK, so let's plug in the #s:

    Your source is:

    16:9 DAR, with H rez of 1024 and V rez of 576, right? So that's

    1.77777777 = (1024 / 576) * ?, or
    ?= 1.7777777 * 576 / 1024 = PAR = 1 (aka square pixels)

    You want -
    4:3 DAR, with H rez of 1024 and V rez of 576, so that's

    1.33333333 = (1024 / 576) * ?, or
    ? = 1.3333333 * 576 / 1024 = PAR = 0.75 or 3/4 or 3:4 depending on which look your setup needs.

    This is the fundamental formula where hello_hello got his calculation from. Personally, I wouldn't worry about ITU vs. MPEG sampling standards here, as the square pixel 1024x576 equivalent is really a bastardization of either, so it shouldn't matter too much.

    However, as ndjamena pointed out, it depends greatly on what the app really means regarding those "MP4 Pixel Aspect Ratio" boxes.

    You could always try it multiple ways and see which looks correct.

    Scott
    "When will the rhetorical questions end?!" - George Carlin
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  11. Member
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    Thank you all very much for your input.

    It's a mystery to me what the program thinks 'Pixel Aspect Ratio' mean.

    The first thing I did before I posted my help-request was putting the PAR at 4:3. This appeared to be a failure.

    Then I got lost.

    But now, by accident, I found I got a perfect 4:3 picture by mirroring the input: 3:4, or 100:75, or 10:13.

    Is this a bug in the software? Or is there a logical explanation for this?

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  12. Member Cornucopia's Avatar
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    No idea. Not logical to me, but then I didn't program that app.

    @HitTheRoad, those 2 pairs of #s are 2.5% different, which may be enough to notice. 3:4 (aka 75:100) ought to be the more accurate of the 2, assuming the original was correctly & exaclty 16:9 square pixels to begin with.

    @smrpix, those figures are appropriate for Standard Non-Square pixel 704x576 or 720x576 PAL video (either in 4:3 DAR or 16:9 DAR respectively), but they wouldn't be appropriate for a video with 1024x576 Square pixel dimensions such as the OP had, assuming the OP just wanted to re-flag the original without re-encoding.

    Scott
    "When will the rhetorical questions end?!" - George Carlin
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  13. Member
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    3:4 is the ratio of each individual pixel, not the picture as a whole. It's what X264 calls the SAR. You have a video that is natively encoded at 1024x576 or 16:9 yet you want to display it at 768x576. To do that you need to squish the picture horizontally, ultimately leading to each pixel being 25% narrower than it is tall ie in order to get a 4:3 picture from a 16:9 block of pixels each pixel must be 3 units wide to 4 units high or 3:4.
    Last edited by ndjamena; 11th Jun 2014 at 07:29.
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  14. Originally Posted by HitTheRoad View Post
    But now, by accident, I found I got a perfect 4:3 picture by mirroring the input: 3:4, or 100:75, or 10:13.

    Is this a bug in the software? Or is there a logical explanation for this?
    It's the shape of the pixels. If the pixels are square (which they were originally), the video will display at 1024x576 (16:9), because it's 1024 pixels wide and 576 pixels high. You want it to display at 768x576, so the pixels need to be "thinner" than they are tall.

    100:75 wouldn't do it. It'd make the picture wider.
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  15. Member
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    Thanks everybody for the explanation!
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